In order to satisfy safety standards and the Electro Magnetic Compatibility (EMC) requirements, safety capacitors are connected between direct current (DC) input terminals as well as alternating current (AC) output terminals of a grid-connected inverter and ground at one or more positions. FIG. 1 is a schematic diagram of equivalent insulation resistors relative to ground and equivalent safety capacitors relative to ground at the DC side as well as the AC side of the grid-connected inverter, where R+ represents an equivalent insulation resistor of a positive DC input terminal relative to ground, R− represents an equivalent insulation resistor of a negative DC input terminal relative to ground, Y+ represents an equivalent Y capacitor of the positive DC input terminal relative to ground, Y− represents an equivalent Y capacitor of the negative DC input terminal relative to ground, and Y1, Y2, Y3 and YN respectively represent equivalent Y capacitors of three-phase live lines U, V, and W and an N line relative to ground.
A DC side and an AC side of an existing grid-connected inverter generally adopt a non-isolated sampling solution, and such non-isolated sampling solution may cause a common-mode loop (as indicated by the dotted lines in FIG. 1) to be formed by a DC side non-isolated sampling network, an AC side non-isolated sampling network and their respective safety capacitors relative to ground. In this case, during maintenance of the grid-connected inverter, although an AC switch for connecting the inverter with the power grid is turned off, the maintenance technician is still exposed to a risk of electric shock since the equivalent Y capacitor relative to ground at the AC side is charged through the common-mode loop, and a significant great common-mode voltage is generated between an AC output cable of the inverter and ground. In addition, most of conventional technologies focus on suppression of a high-frequency common-mode voltage in the voltage in grid-connected state. However, during maintenance of the inverter when the AC switch is turned off, a significant common-mode DC voltage on the AC cable generated due to the existence of the common-mode loop still causes an electric shock risk to the maintenance technician.
Further, in consideration that reference grounds PEs of the DC side as well as the AC side of the inverter are metal housing of cases, and equivalent Y capacitors relative to ground are distributed in different positions, it is difficult to cut off the common-mode loop by directly connecting switches in series. In the conventional technology, there are some solutions where the common-mode loop is cut off using the DC side isolated sampling network and the AC side isolated sampling network, thereby eliminating the common-mode voltage of the AC cable relative to ground, as shown in FIG. 2. However, a system adopting such a solution has a high implementation cost, and has a high requirement on the insulation class of the components of the isolated sampling network.